Generally, a fuel cell is a device which converts the energy of a chemical reaction into electricity. Fuel cells differ from batteries in that fuel and oxidant are stored external to the cell, which can generate power as long as the fuel and oxidant are supplied. A fuel cell produces an electromotive force by bringing the fuel and oxidant in contact with two suitable electrodes separated by an electrolyte. A fuel, such as hydrogen gas, is introduced at one electrode where it dissociates on the electrocatalytic surface of the positive electrode (anode) to form protons and electrons, as elucidated in equation 1. The electrons pass into the conductive structure of the electrode, and there from to the external electrical circuit energized by said fuel cell. The protons formed by dissociation of the hydrogen at the first electrode and pass through the electrolyte to the second electrode. Simultaneously, an oxidant, such as oxygen gas or air, is introduced to the second electrode where it is adsorbed on the electrocatalytic surface of the negative electrode (cathode) and is electrochemically reduced to form a surface oxide species by electrons having transversed the external electrical circuit energized by the fuel cell. This surface oxide reacts with protons from the electrolyte to form water, the product of the net reaction. The water desorbs from the electrode and leaves the cell in the cathode gas stream. The half cell reactions for a hydrogen consuming fuel cell at the two electrodes are, respectively, as follows: EQU H.sub.2 2H.sup.+ +2e.sup.- (1) EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.- H.sub.2 O (2)
Connecting the two electrodes through an external circuit causes an electrical current to flow in the circuit and withdraws electrical power from the cell. The overall fuel cell reaction, which is the sum of the separate half cell reactions written above, produces electrical energy and heat.
In practice, fuel cells are not operated as single units, but are connected in a series to additively combine the individual cell potentials and achieve a greater, and more useful, potential. The cells in a given series can be connected directly, with opposing faces of a single component in contact with the anode of one cell and the cathode of an adjacent cell, or through an external electrical linkage. A series of fuel cells, referred to as a fuel cell stack, are normally equipped with a manifold system for the distribution of two gases. The fuel and oxidant are directed with manifolds to the correct electrodes, and cooling is provided either by the reactants or by a cooling medium. Also within the stack are current collectors, cell-to-cell seals, and other components. The stack and associated hardware make up the fuel cell module.
In fuel cells where a solid polymer electrolyte or proton exchange membrane ("PEM") is used, the membrane acts as the electrolyte as well as a barrier for preventing the mixing of the reactant gases. A PEM fuel cell is described in greater detail in Dhar, U.S. Pat. No. 5,242,764, which is incorporated herein by reference. Much research and development has been devoted to improving the power-to-weight ratio for proton exchange membrane ("PEM") fuel cells. Most of this research has involved increasing the power per unit volume of relatively heavy stacks.
FIG. 1 is a drawing illustrating a fuel cell stack based on a conventional bipolar filter press design 10 with graphite structure elements. A full description of filter press type fuel cells may be found in Neidrach, U.S. Pat. No. 3,134,697 which is incorporated herein by reference. While improvements in the filter press style fuel cells have provided significant increases in power per unit volume, the overall systems that have evolved are large, heavy, and relatively complex, with compressors to supply air and pumps to provide forced water cooling systems to remove excess heat.
FIG. 2 shows the structure of a standard fuel cell membrane and electrode (M&E) assembly 20 intended for use in the bipolar stack 10 of FIG. 1, which has current collection over most of the back of the electrode. The M&E assembly consists of a membrane 22, a catalyst layer 24, a gas diffusion layer 26 and a conductive cloth backing 28. As illustrated, a complete M&E assembly includes similar layers formed on both sides of the membrane.
More recently, efforts have been made to reduce the stack weight by replacing the heavy carbon elements with thinner and lighter, metal elements. However, these units were designed for large scale applications, some on the order of about 30 kW, and, therefore, require the same stack ancillary equipment mentioned above. Furthermore, the ancillary equipment included with the stack in these systems has been designed to operate efficiently at the kilowatt level. Scaled down versions of these systems have been attempted in applications that require much less power, such as within the range between about 50 and about 150 Watts. However, these systems are not well suited for stack outputs in the tens or hundreds of watts, since the rotating components, such as pumps and compressors, do not scale down well. As a result, even small scale systems of this design are too heavy for many small applications, such as for portable applications and personal use.
Therefore, perhaps the most important objective for portable and personal applications is not Watts per unit volume but Watts per unit weight, i.e. W/lb. Efforts to adapt the standard bipolar filter press design to low pressure operation, thereby eliminating much of the ancillary equipment, have met with some limited success, producing stacks with power densities as high as 61 W/lb. While this is a useful power density, these systems require complicated and expensive assembly.
One possible way of improving fuel cell systems for operation at lower pressures is using liquid fuels in lieu of gaseous fuels, such as hydrogen. Methanol (CH.sub.3 OH) and other related compounds, such as dimethoxymethane (C.sub.3 H.sub.8 O.sub.2) and trimethoxymethane (C.sub.4 H.sub.10 O.sub.3), offer very promising alternatives to gaseous fuels.
The liquid fuels mentioned above share some common advantages compared to hydrogen. First, they are all pourable liquids at ambient pressure and ambient, and near ambient, temperatures. Second, they have a much higher energy density than hydrogen. For example, a 1:1 methanol:water mixture (each mole of methanol requires a mole of water for electrochemical oxidation, as shown in Equation 3) has as much potential energy as hydrogen stored at a pressure of 16,000 psi. EQU CH.sub.3 OH+H.sub.2 O+3/2O.sub.2 .fwdarw.CO.sub.2 +3H.sub.2 O(3)
Although there is a net production of water, e.g., more water is produced at the cathode than consumed at the anode, water must be supplied to the anode because in a fuel cell the oxidation is carried out as a pair of half cell reactions, with Equation 3 representing the net reaction. In a fuel cell water is consumed in the anode reaction (Equation 4) and produced in the cathode reaction (Equation 5). EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +6H.sup.+ +6e.sup.- (4) EQU 6H.sup.+ +6e.sup.- +3/2O.sub.2 .fwdarw.3H.sub.2 O (5)
The reactions for dimethoxymethane and trimethoxymethane are similar, with four and five molecules of water needed for each molecule of the oxidized organic compound respectively. Since they are consumed at higher water-to-fuel stoichiometries, the two ethers should be present at lower concentrations in the fuel stream, and consequently have lower permeation rates through PEM membranes and lower vapor pressures over the solution. This is especially true in the case of trimethoxymethane, which has a boiling point of 104.degree. C., 40.degree. C. higher than methanol, and consequently, a lower vapor pressure than methanol under all conditions. The lower vapor pressure, combined with lower toxicity for these compounds, leads to less vapor toxicity hazards compared to those associated with the use of methanol.
Therefore, there is a need for a lightweight fuel cell system that provides an improved power density (W/lb) and eliminates much of the ancillary equipment. There is also a need for fuel cells that operate on gaseous fuels, such as hydrogen, and fuel cells that operate on liquid fuels, such as dimethoxymethane. It would be desirable if the fuel cell operated efficiently in the 50 to 150 Watt range to supply electricity to a variety of common electrical devices. It would also be desirable if the fuel cell had no more than a few moving parts to reduce maintenance and avoid breakdowns. It would be further desirable to have a fuel cell system that was available in modules that could be configured together to meet the power requirements of specific applications.